Novel catalyst for the reduction of NO to N2 with hydrogen under NOx oxidation conditions

ABSTRACT

The invention relates to a novel catalyst having excellent activity, selectivity and stability for reducing nitric oxide to gas nitrogen, with hydrogen being used as a reducing agent, in the low temperature range 100-200 ° C. and in the presence of an excess of oxygen (e.g. 5% vol), H 2 O (5% vol) and/or SO 2  (20 ppm) in the supply. The inventive catalyst consists of platinum crystals which are in contact with the phases of a mixed MgO and CeO 2  medium or in the form of platinum supported on the mixed MgO—CeO 2  medium which has already been sulphated in a selective manner. The Pt/MgO—CeO 2  catalyst can be used to obtain NO conversion levels which are greater than 40% and nitrogen selectivity values of greater than 80% under NOx oxidation conditions in the 100-400° C. temperature range and for a surface contact time of 0.045 s. In particular, full NO conversion is obtained as well as N 2  selectivity levels of 83% at 150° C. with a reaction mixture of 0.25% NO/1% H 2 /5% O 2 /5% H 2 O/He.

RELATED APPLICATIONS

The present application is a Continuation of co-pending PCT ApplicationNo. PCT/ES03/00083, filed Feb. 14, 2003 which in turn, claims priorityfrom Spanish Application Serial No. P200200368, filed on Feb. 15, 2002.Applicants claim the benefits of 35 U.S.C. §120 as to the PCTapplication and priority under 35 U.S.C. §119 as to said Spanishapplication, and the entire disclosures of both applications areincorporated herein in their entireties.

FIELD OF THE TECHNIQUE

This invention refers to a novel catalyst based on platinum, withexcellent activity, stability and selectivity for reducing NO to N₂ byusing H₂ as reducing agent in the low temperature range 100-200° C. andin the presence of an excess of oxygen (e.g. 5% vol), 5% vol H₂O and/or20 ppm SO₂ in the reactor supply.

This catalyst can be used in the selective conversion of nitric oxide,produced in many industrial combustion processes, to N₂ gas. It is knownthat hydrogen is available in numerous industrial installations. Usingthe said catalyst, just a very small percentage of the availablehydrogen is necessary for the reduction of NO to N₂ under clearlyoxidizing conditions of NOx in the low temperature range 100-200° C.

The selective catalytic reduction of NO with NH₃ (NH₃—SCR) in thepresence of an excess of oxygen has aroused great interest in the lastthree decades and has recently been reported and revised in theliterature [1]. In this process, ammonia is used as reducing agent ofnitrogen oxides and nitrogen and water are produced as the reactionproducts. Vanadium pentoxide (V₂O₅) supported on oxides such as TiO₂,Al₂O₃ and SiO₂ and promoted with WO₃ constitutes an efficient catalystused in industry [2]. This catalyst is active in the temperature range250-550° C. [3]. Nevertheless, the toxicity and problems of handlingammonia [1,4] constitute the main obstacles preventing the use of thistechnology by the general public, and at the same time it seems thatproblems deriving from spillages of NH₃ and corrosion and poisoning ofthe catalyst by SO₂ cannot be solved with current technology. Theautomobile industry never applied the NH₃—SCR process. This is primarilydue to the absence of NH₃ in the exhaust gases of automobiles and themarked deactivation of the NH₃—SCR catalyst (V₂O₅—WO3/TiO₂) in thepresence of excess oxygen in the reaction stream [5] (as in the case ofautomobiles with diesel engines).

Nevertheless, the NH₃—SCR process is the best catalytic technology knownfor the elimination of NO in stationary sources and it is used as adecontamination process primarily in conventional thermal powerstations.

The selective catalytic reduction of NO with hydrocarbons (HC—SCR) hasbeen exhaustively studied in recent years as a potential competitor ofthe NH₃—SCR process [6,7]. The main advantage of this catalytic reactionis the potential use of hydrocarbons as reducing species that can befound in the exhaust gases of combustion processes operating underclearly oxidizing conditions of NOx. The catalysts that have attractedthe attention for the HC—SCR process of NO can be divided into threemain groups: (a) supported noble metals; (b) zeolites exchanged withmetal ions; and (c) metal oxide catalysts [3]. Among these materials,supported noble metals have shown the best and only catalytic behaviourfor the reduction of NO with hydrocarbons under oxidation conditions atreaction temperatures as low as 120-2502° C. [8-16]. Also, it was foundthat these catalysts are more resistant to deactivation in the presenceof water and/or SO₂ [17,18]. Nevertheless, in spite of their exceptionalactivity in this low temperature region, supported Pt and Pd catalystspresent low values of selectivity towards N₂ [19,20] and a relativelynarrow range of operating temperatures. On the contrary, zeolitesexchanged with metal ions are very active and selective for the SCR ofNO with hydrocarbons at relatively low temperatures. However, thesecatalysts present an even narrower operating temperature range comparedto that of supported noble metals. Also, the activity of zeoliticcatalysts exchanged with metal ions drastically diminishes in thepresence of water. Finally, catalysts based on metal oxides showed lowactivity but high selectivity towards N₂ for HC—SCR processes of NO butat temperatures above 500° C.

Current concerns regarding carbon dioxide emissions into the atmosphereand the problems resulting from the use of NH₃ as reducing agent [21]have encouraged a search for suitable molecules different fromhydrocarbons for the catalytic reduction of NO in gaseous currentscoming from combustion. It has been reported that hydrogen is a veryenergetic reducing agent for the reaction NO/H₂ [22-33] and canpotentially be used for reducing NO_(x) emissions coming from stationarycombustion sources. Hydrogen is currently used in industrial processesof petroleum refining such as hydrotreatment and hydrocracking [34-36],the production of methanol [37,38], the conversion of methanol togasoline [39,40] and the synthesis of ammonia [41,42] and hydrocarbons(Fischer-Tropsch process) [43-45]. So, hydrogen is available in manyindustrial installations wherein various processes are operatedrequiring a heat input. To this must be added the progressive demand forhydrogen with a growth rate of approximately 10% a year [46], whichmeans that the availability in the industrial sector will be increasingfurther in the coming years.

Therefore, an H₂—SCR catalytic technology of NOx can be considered as animportant qualitative leap compared to NH₃—SCR and HC—SCR catalyticprocesses.

It is important to mention here that in the absence of oxygen in thesupply stream hydrogen cannot be regarded as a selective reducing agentdue to the fact that as well as nitrogen, other undesired products areusually produced such as N₂O and NH₃. Just a few attempts have beenreported to reduce NO with H₂ under oxygen-rich conditions [47-51] andthis is due to the fact that hydrogen displays high combustionvelocities with O₂ for forming H₂O under the applied reactionconditions. The strong competition among species of NOx and oxygenadsorbed by the hydrogen under the applied reaction conditions [52,53]makes the development of suitable catalytic systems a difficult task.

It has been found that platinum supported catalysts, such as Pt/Al₂O₃and Pt/SiO₂, are the most active for the reaction NO/H₂/O₂ under clearlyoxidizing conditions of NOx at low temperature (T<200° C.) [47-51].Yokota et al [50] reported catalytic activity results in the reductionof NO with H₂ in the presence of O₂ on a Pt—Mo—Na/SiO₂ catalyst, whileFrank et al [47] reported kinetic results of the reaction NO/H₂/O₂ on aPt—Mo—Co/α-Al₂O₃ catalyst. The last two catalysts produced substantiallylower quantities of N₂O (selectivity to N₂ close to 75%) than theconventional Pt supported catalysts (e.g., Pt/Al₂O₃, SiO₂), whichpresent lower selectivity levels to N₂ (S_(N2)=40-60%). On the otherhand, it has been documented [50] that the catalyst Pt—Mo—Na/SiO₂presents a relatively low stability under clearly oxidizing conditionsin the presence of water, while the catalysts Pt—Mo—Na—/SiO₂ andPt—Mo—Co/α-Al₂O₃ present a relatively narrow operating temperaturerange. In a previous work [54] we have reported reduction results of NOwith H₂ in the presence of excess O₂, obtained on a platinum catalystsupported on a perovskite type substrate. This catalyst turned out to bethe most active and selective of the platinum supported catalystsreported in the literature for the reaction NO/H₂/O₂ up to the date ofpublication. In spite of the fact that the catalyst Pt/La—Ce—Mn—O [54]turned out to be very active and selective in a wide temperature rangecompared to other platinum based catalysts, this operating temperaturerange becomes much lower than that shown by the catalyst Pt/MgO—CeO₂.

The results described above reflect the general agreement of scientiststhat the support has a crucial effect on the activity and selectivity ofplatinum supported catalysts in the reduction of NO with H₂ in thepresence of an excess of oxygen [7].

On the basis of everything that has been stated, it is of industrialinterest to develop an improved catalyst based on platinum with thefollowing characteristics for the reaction NO/H₂/O₂:

-   -   (a) High activity and selectivity at low reaction temperatures        (e.g. below 200° C.) with N₂ yields greater than 90%.    -   (b) A broad operating temperature range (e.g. 100-200° C.) with        appreciable values of NO conversion and selectivity to N₂.    -   (c) Prolonged stability during the course of the operation.    -   (d) Stability in the presence of at least 5% vol H₂O and SO₂ in        the range 1-20 ppm.

DESCRIPTION OF THE INVENTION

This invention describes a novel catalyst based on platinum, withexcellent activity, selectivity and stability for reducing nitric oxideusing hydrogen as reducing agent in the low temperature range 100-200°C. and in the presence of an excess of oxygen. The catalyst consists ofplatinum crystals in contact with the two phases of MgO and CeO₂ or inthe form of platinum supported on a MgO—CeO₂ mixed oxide supportpreviously sulphated in a selective manner. Prior to the impregnation ofthe oxide phases with the platinum precursor, the pre-sulphation of thesupport (50% MgO—CeO₂) is necessary. This is achieved by impregnation ofthe support with an aqueous solution of NH₄NO₃ followed by (NH₄)₂SO₄ asdescribed in Example 1. Calcination of the resulting solid in air at600° C. for at least 2 h is necessary for the complete elimination ofthe ammonium cation and stabilization of the surface structure of thesupport. The catalyst 0.1% wt Pt/50% MgO—CeO₂ can be prepared by any ofthe means known by practitioners of this art, including the technique ofdamp impregnation of the pre-sulphated support with an aqueous solutionof the Pt precursor (e.g., solution of hexachloroplatinic acid(H₂PtCl₆)). Following the preparation of the Pt supported catalyst atleast 2 h of calcination in air at 600° C. are necessary for thecomplete transformation of the platinum precursor into platinum oxide.Finally, a reduction has to be carried out with hydrogen at 300° C. forat least 2 h in order to fully reduce the platinum oxide to metallicplatinum. The resulting catalytic surface is very stable, without anydeactivation being observed during 24 h of reaction or more, even in thepresence of 5% vol H₂O and/or 20 ppm SO₂. Hereinafter, the catalystdescribed above will be known as Pt/s-MgO—CeO₂ where s indicates thesulphated support 50% MgO—CeO₂. Virtually complete conversion of NO isobtained at 150° C. on this catalyst in a contact time of 0.045 s. Theindustrial reactors of NH₃—SCR of NO which use industrial catalystsoperate under typical surface contact times of 0.08-0.4 s [1,55-57]. Bymeans of applying this new catalyst based on Pt (e.g., 0.1% wt Pt/s-50%MgO—CeO₂) the conversion of NO to N₂ with H₂ under clearly oxidizingconditions of NOx can be considered at a broader scale.

This invention describes a novel catalyst based on platinum, withexcellent activity, selectivity and stability for reducing nitric oxideto nitrogen using hydrogen as reducing agent in the low temperaturerange 100-200° C. and in the presence of an excess of oxygen, 5% vol H₂Oand/or 20 ppm SO₂ in the supply. The catalyst was prepared by the dampimpregnation method previously described above. Identical catalysts canbe prepared using other preparation techniques known by practitioners ofthis art, and other metallic precursors such as platinum nitrate,platinum acetyl-acetonate, platinum chloride, etc. Nevertheless, it hasbeen found in this work that the preparation of the catalyst mentionedabove using the sol-gel method [58] provided better results in terms ofcatalytic activity and selectivity to N₂ in the reaction NO/H₂/O₂ (seeFIG. 3). Eight different mixtures of MgO—CeO₂ were used as supports witha magnesium content (x % wt MgO) varying from 0 to 100%.

In this work it was found that the pre-sulphated MgO—CeO₂ mixed oxide(see Example 1) is essential for achieving high stability towardsdeactivation by SO₂ (Example 8, FIG. 7). It must be noted here that thenon-sulphated catalyst 0.1% wt Pt/MgO—CeO₂ showed selectivity valuestowards N₂ in the range 65-72%, while the pre-sulphated catalyst showedhigher selectivity values towards N₂ (greater than 80%, see Example 6and FIG. 5).

In this work it was also found that the nature of the support has alarge effect on the activity and selectivity of the corresponding Ptsupported catalyst. While Pt supported on SiO₂ presents selectivityvalues lower than 60% [59], the catalyst Pt/s-50% MgO—CeO₂ exhibitsselectivities higher than 80%. Also, the integral production velocity ofnitrogen obtained on the catalyst Pt/s-50% MgO—CeO₂ is slightly higherthan that found with the catalyst Pt/La_(0.5)Ce_(0.5)MnO₃ [54] thoughclose to 50% higher than that obtained on Pt/SiO₂ [59]. Nevertheless,the catalyst Pt/s-50% MgO—CeO₂ presents an extraordinarily broadoperating temperature range (ΔT, see Table 1), much broader than thatobtained on the catalysts Pt/SiO₂ and Pt/La_(0.5)Ce_(0.5)MnO₃. It mustbe noted that the last catalyst is the most active and selective of allthe ones reported for the reaction NO/H₂/O₂ under NOx oxidationconditions [54]. The integral production velocity of nitrogen on thecatalyst Pt/MgO—CeO₂ can even be raised by means of increasing thepartial pressure of hydrogen. In particular, the integral productionvelocity of N₂ on the said catalyst can be raised up to almost fourtimes when the partial pressure of H₂ is increased from 1 to 3% vol at200° C. (Example 9, FIG. 8).

The ratio of MgO to CeO₂ is an important factor which affects thecatalytic behaviour (reaction velocity and selectivity) of the catalystPt/MgO—CeO₂. It is shown (see Example 2, FIG. 1) that the catalyst witha weight ratio of MgO to CeO₂ equal to one presents the highest integralproduction velocity of N₂ at both low and high reaction temperatures. Asshown in FIG. 1, almost all the compositions present higher velocitiesthan those predicted by the rule of mixture (dotted line) at 150° C.(Example 2, Eq. [1]). So, a positive synergetic effect results.Nevertheless, when the reaction temperature rises to 300° C., thebehaviour of the reaction velocity toward the content of MgO isdifferent (FIG. 1). All the catalysts except Pt/50% MgO—CeO₂ presentexperimental velocities substantially lower than those expected from thelaw of mixture (a negative synergetic effect is seen). In the case ofthe catalyst Pt/50% MgO—CeO₂ an important positive synergetic effect isobtained at both temperatures.

The platinum content of the catalyst Pt/MgO—CeO₂ is a crucial factoraffecting its catalytic behaviour. As was shown in FIG. 2 (see alsoExample 3), the catalyst with least metallic content (0.1% wt) presentsthe highest integral reaction velocity referring to a gram of metallicPt, compared to that of catalysts with higher metallic contents. Theformation reaction velocity of N₂ decreases with an increase in theplatinum content. Since catalysts with low metal contents have highdispersions, it can be concluded that the reduction reaction of NO withH₂ in the presence of excess O₂ occurs favourably on the surface ofsmall metallic particles. Galvano and Paravano reported very similarresults on different gold supported catalysts for the reaction NO/H₂.These authors found that the selectivity of the reaction NO/H₂ towardsN₂ decreased with the increase of the particle size of the gold forcatalysts supported on MgO and Al₂O₃.

The catalyst Pt/s-50% MgO—CeO₂ showed excellent stability with thereaction time in the presence of 5% vol H₂O in the supply (Example 7,FIG. 6), which is higher than that observed with the catalysts Pt/SiO₂and Pt/La_(0.5)Ce_(0.5)MnO₃ previously investigated [54,59]. Constantproduction velocities of N₂ were observed even after 24 h in current onthe catalyst 0.1% wt Pt/s-50% MgO—CeO₂. On the other hand, the integralproduction velocity of N₂ obtained with the catalystsPt/La_(0.5)Ce_(0.5)MnO₃ and Pt/SiO₂ fell substantially during the first2 hours in current and continued to fall, though more slowly, duringlonger times in stream (FIG. 6). This is typical behaviour of many NOxcatalysts that have been reported when water is present in the supplystream [3], which means that the catalytic stability results reportedhere are of major practical importance.

As noted above, the fresh catalyst Pt/MgO—CeO₂ becomes deactivated inthe presence of 20 ppm SO₂ in the supply stream (FIG. 7). This is awell-known phenomenon in NOx catalysts. The deactivation of the catalystPt/MgO—CeO₂ probably occurs by adsorption and reaction of gaseous SO₂with the oxide phases of the catalyst, giving rise to a progressivesulphation of the support (e.g., MgSO₄, Ce₂(SO₄)₃). These processes inturn probably cause an irreversible poisoning of the active centres dueto the formation of nitrate/nitrite according to the literature [51,61].Nevertheless, when the support MgO—CeO₂ is pre-sulphated beforehand (seeExample 1 and FIG. 7) the catalyst Pt/s-MgO—CeO₂ presents excellentstability in the presence of SO₂. This implies that during thepre-sulphation of the support, the nitrate/nitrite formation centres donot become poisoned and the adsorption of SO₂ occurs selectively oncentres that are non-active for the reaction NO/H₂/O₂ under NOxoxidation conditions. Moreover, it is possible that the adsorption ofSO₂ in those centres also inhibits the later sulphation of the supportand the development of the crystalline phases MgSO₄ and/or Ce₂(SO₄)₃under reaction conditions. The effective sulphation of the solidMgO—CeO₂ can also be assured by damp impregnation of the original samplewith a nitrate solution. The results obtained in this case are the sameas those described above. Hodjati et al [62] also reported a similarbehaviour on a catalyst of NOx BaSnO₃.

The present invention, e.g., the pre-sulphated catalyst 0.1% Pt/s-50%MgO—CeO₂, is a novel catalyst wherein the main differences with respectto catalysts based on noble metals and other catalysts of NOx reportedfor the reaction NO/H₂/O₂ are as follows:

Catalysts based on noble metals have a high cost and limitedavailability of the noble metal. Nevertheless, owing to the highactivity of the new catalyst Pt/s-MgO—CeO₂ (Table 1) much lower platinumcontents can be used (e.g., 0.1% wt) instead of higher noble metalcontent (e.g. 1% wt) normally used in industrial NOx applications. So,the cost of this catalytic system can be substantially reduced.

In spite of the fact that catalysts based on noble metals are less proneto becoming deactivated in the presence of water and/or SO_(2 [17,18)],such catalysts have not been reported to be stable in the presence ofwater and/or SO₂ in the reaction NO/H₂/O₂ under NOx oxidationconditions. Nevertheless, the new catalyst Pt/s-MgO—CeO₂ is very stablein the presence of 5% vol water or 20 ppm SO₂ if the actual sulphationmethod is followed (Example 1, FIG. 8).

Metal oxide catalysts present high selectivity levels to N₂ in thereaction NO/H₂/O₂, very similar to those obtained with the new catalystPt/s-MgO—CeO₂. Nevertheless, oxide catalysts are much less active whencompared with the latter noble metal catalyst. Also, metal oxidecatalysts are active only at temperatures higher than 400 ²C while thecatalyst Pt/s-MgO—CeO₂ presents a conversion maximum of NO at 150° C.So, metal oxide catalysts cannot be regarded as candidates for NOxapplications under low temperature oxidation conditions.

Zeolites exchanged with metallic ions are very active and selective forthe SCR of NO with hydrocarbons at relatively low temperatures.Nevertheless, the catalysts present a very narrow operating temperaturerange compared to the new catalyst Pt/s-MgO—CeO₂. Moreover, the activityof catalysts exchanged with metallic ions falls sharply in the presenceof water and/or SO₂, while the new catalyst Pt/s-MgO—CeO₂ remains stablein the presence of water or SO₂.

The new catalyst Pt/s-MgO—CeO₂ is the most active, selective and stablereported to date for the reaction NO/H₂/O₂ under NOx oxidationconditions. Also, this catalyst presents the broadest window ofoperating temperatures reported for the said reaction (Example 5, Table1).

NH₃—SCR is widely used as anti-contamination technology for theelimination of NO from stationary sources, mainly in conventionalthermal power stations [1]. On the other hand, the problems of toxicityand handling of ammonia [1,4] constitute major obstacles against the useof this technology by the general public. In addition, problems relatedwith NH₃ corrosion and poisoning of catalysts by SO₂ seem difficult tosolve. Yet, an SCR technology for NO_(x) based on hydrogen (H₂—SCR) caneliminate most of the problems that have been enumerated.

Hydrogen is widely used in industry [34-46]. In fact, the availabilityof hydrogen in industry is much greater compared to that of ammonia[46].

The differences discussed above mean that H₂—SCR catalytic technologyfor NO_(x) of the present invention is new and innovative.

EXAMPLE OF EMBODIMENT OF THE INVENTION

The following examples represent a more detailed description of theinvention. There can be no doubt that this detailed description is madeby way of illustration only and does not limit the extent of theinvention since there are many variations that can be made to it withoutdetracting from the spirit of this invention.

Example 1

This example illustrates the synthesis of platinum-based catalysts,supported on a MgO—CeO₂ mixed oxide. Pre-sulphated Pt/s-MgO—CeO₂catalysts were prepared by means of the damp impregnation method asfollows:

1 g of MgO (Aldrich 34,279-3, 99+%) and 1 g of CeO₂ (Aldrich 34,295-5,99.9%) were impregnated with 50 ml of an aqueous solution containing 7.1mg (90 μmols) of NH₄NO₃ (Aldrich, ultra-pure). The water was evaporatedwith continuous stirring and the residue was dried at 100° C. for 4 h.The residue was then sieved and heated at 300° C. in the presence of airfor 2 h in order to fully decompose the ammonium cations. This processwas followed in order to ensure the protection (of the sulphation) ofthe centres for the adsorption of nitrate by the support. The resultingsolid was impregnated with 50 ml of an aqueous solution containing 24 mg(90 μmols) of (NH₄)₂SO₄ (Aldrich, ultra-pure). The water was thenevaporated with continuous stirring and the residue was dried at 100° C.for 4 h. The residue was sieved and heated in air at 600° C. for 2 h andthen cooled to room temperature. 2 g of sulphated support were thenimpregnated with an aqueous solution containing the desired quantity ofhexachloroplatinic acid (Aldrich, 26,258-7). The excess of water wasevaporated with continuous stirring and the residue was dried at 80° C.for 24 h. The dry residue was sieved and heated at 600° C. in a flow ofair for at least 2 h in order to completely decompose thehexachloroplatinic acid. The catalyst was then reduced in a flow of H₂at 300° C. for at least 2 h. The content of metallic platinum varied inthe range 0.1-0.2% by weight.

Two catalysts 0.5% wt Pt/50% MgO—CeO₂ were also prepared by the sol-gelprocedure following the experimental conditions described byBalakrishnan et al [58]. Weighed quantities of Mg(OEt)₂, Ce(NO₃)₃ andPt(NH₃)₂(NO₃)₂ were dissolved in a solution of EtOH/H₂O followed bycontinuous stirring and heating at 60° C. until a gel forms. The mixedoxide support 50% wt MgO—CeO₂ and was also prepared by the ceramicmethod [63] using pure oxides as starting materials. 0.5% wt of Pt wasthen deposited on the resulting solid by means of damp impregnation.

The dispersion of platinum in the Pt/MgO—CeO₂ catalysts was measured bymeans of H₂ chemisorption at 25° C. following by thermal programmeddesorption (TPD) in a He flow. Prior to the TPD of the H₂, the samplewas purged in He for 45 min at room temperature. A dispersion ofplatinum of 83% in the 0.1% wt Pt/50% MgO—CeO₂ catalyst was determined.

Example 2

The integral nitrogen production velocities were determined for thereaction on platinum supported catalysts in the range 100-400° C. asfollows:

150 mg of sample of catalyst were placed in a fixed bed quartzmicro-reactor. The reactor supply consisted of 0.25% vol NO, 1% vol H₂,5% vol O₂ and 93.75% vol He. A flow velocity of 100 ml(STP)/min was usedgiving a GHSV of approximately 80,000 h⁻¹. FIG. 1 shows the effect ofthe content of MgO (x, % wt) in the mixture 0.5% wt Pt/MgO—CeO₂ on theproduction velocity of N₂ per gram of catalyst obtained during thereaction NO/H₂/O₂ at 150 and 300° C. It can be clearly seen in FIG. 1that the Pt supported catalyst with a MgO content of 50 wt % presentshigher N₂ production velocities at the two stated temperatures. Thedotted lines represent the N₂ production velocities predicted by therule of mixture given by the following equation:R _(m)=(x/100).R _(Mgo)+(1−(x/100)).R _(CeO2) (μmols/g.s) [1]

Equation [1] permits the reaction velocity (R_(m)) to be calculated forthe two catalytic phases of Pt/CeO₂ and Pt/MgO based on the individualvelocities of each catalytic phase and the content x % wt of the phasein the mixture. If there does not exist any cooperation (synergy)between the two phases, then the experimental reaction velocity observedon the mixture of the two solids must also be predicted by Eq [1]. Ascan be seen in FIG. 1, all the catalysts except 0.5% wt Pt/90% MgO—CeO₂present higher velocities than those predicted by the rule of mixture(Eq [1]) at 150° C. So, a positive synergetic effect is produced.Nevertheless, when the reaction temperature increases to 300° C. adifferent behaviour is seen in the reaction velocity regarding thecontent of MgO (FIG. 1). All the catalysts except for Pt/50% MgO—CeO₂present substantially lower experimental velocities than those predictedby the rule of mixture (a negative synergetic effect is observed). Thecase of the catalyst 0.5% wt Pt/50% MgO—CeO₂ can be stated, whichpresents a positive synergetic effect at both temperatures.

Example 3

In this example, the influence of the platinum content on the integralN₂ production velocity was investigated in the range 100-400° C. on thesystem Pt/50% MgO—CeO₂.

The supply consisted of 0.25% vol NO, 1% vol H₂, 5% vol O₂ and 93.75%vol He. 100 mg of each sample was used for these experiments, while thetotal flow velocity was kept at 100 ml(STP)/min giving a GHSV ofapproximately 120,000 h⁻¹. FIG. 2 presents the integral N₂ productionvelocities per gram of total platinum on the sample as a function oftemperature for five platinum contents: 0.1, 0.3, 0.5, 1 and 2% wt Ascan be seen in FIG. 2, the N₂ production velocity falls drastically withthe increase in platinum content at any temperature in the range100-400° C. So, it can be said that the reduction of NO with H₂ in thepresence of excess O₂ on the catalytic system Pt/50% MgO—CeO₂ isfavoured at low platinum contents.

Example 4

This example illustrates the effect of the preparation method on thetemperature profile of the N₂ integral production velocity for thereaction NO/H₂/O₂ under NOx oxidation conditions on the catalysts 0.5%wt Pt/50% MgO—CeO₂. 100 mg of each catalyst 0.5% wt Pt/50% MgO—CeO₂,prepared by damp impregnation, sol-gel and ceramic process methods, wereused.

The reaction conditions used in this example are the same as thoseemployed in Example 3. FIG. 3 presents the temperature profile of the N₂integral production velocity per gram of total platinum obtained on thethree catalysts mentioned for the reaction NO/H₂/O₂ in the range100-400° C. It is evident from FIG. 3 that the catalyst 0.5% wt Pt/50%MgO—CeO₂ prepared by the sol-gel method presents substantially higher N₂production velocities in the range 120-200° C. compared to the catalystsprepared by the ceramic and damp impregnation methods. The last twosolids show a very similar catalytic behaviour in the range 100-400° C.So, the sol-gel method is preferred for the preparation of the catalystPt/50% MgO—CeO₂ instead of the ceramic or damp impregnation methodsdescribed previously.

Table 1 below compiles the catalytic behaviour of various Pt supportedcatalysts for the reaction NO/H₂/O₂ under NOx oxidation conditionsreported in the open literature. The corresponding results obtained withthe catalyst 0.1% wt Pt/s-50% MgO—CeO₂ for the said reaction are alsoincluded in Table 1. In this table, ΔT is the temperature range whereinX_(NO) is greater than ½ of the maximum observed conversion of NO. Thelatter parameter could be used for defining the quality of the operationtemperature window. For example, a high value of ΔT corresponds to thebest desired operation of the catalyst under practical conditions. Table1 also compiles the integral N₂ production velocity per gram of total Pt(R_(N2)) evaluated in accordance with the obtained values of X_(NO) andS_(N2) for each catalyst. Moreover, the mean conversion value of NO(X_(NO)) in the range 100-400° C. is also included in Table 1. Thisparameter was calculated using the following formula: $\begin{matrix}{{\overset{\_}{X}}_{NO} = {\frac{\int_{100}^{400}{X_{NO}\quad{\mathbb{d}T}}}{400 - 100} = \frac{\int_{100}^{400}{X_{NO}\quad{\mathbb{d}T}}}{300}}} & (2)\end{matrix}$A similar formula was also used for calculating the mean value ofselectivity to N₂ which is also given in Table 1. $\begin{matrix}{{\overset{\_}{S}}_{N_{2}} = \frac{\int_{T_{1}}^{T_{2}}{S_{N_{2}}\quad{\mathbb{d}T}}}{T_{2} - T_{1}}} & (3)\end{matrix}$In Eq [3], T₁ and T₂ are the highest and lowest temperaturesrespectively where catalytic activity can be measured.

In accordance with the results of Table 1, the present catalyst 0.1% wtPt/s-50% MgO—CeO₂ is the best in terms of catalytic behaviour of all thetabulated catalysts. Since the reaction orders with respect to the threereactants must not be greater than 1.5, it is evident from the data ofTable 1 that the catalyst 0.1% wt Pt/s-50% MgO—CeO₂ presents the highestactivity, selectivity and operation temperature window (ΔT) everreported for the reaction NO/H₂/O₂. Nevertheless, the comparison betweenthe catalysts Pt/s-MgO—CeO₂, Pt/La—Ce—Mn—O Pt/Al₂O₃ and Pt/SiO₂ isdirect when the same experimental conditions are used. The meanconversion value of NO increases by approximately 50% when the Pt issupported on s-50% MgO—CeO₂ with respect to the supportLa_(0.5)Ce_(0.5)MnO₃, while the increase becomes larger (230%) if thecomparison is made with the support SiO₂. Also, the mean value ofselectivity to N₂ of 86.5% obtained with the catalyst Pt/s-50% MgO—CeO₂is the same as that obtained with the catalyst Pt/La_(0.5)Ce_(0.5)MnO₃but is much greater than that obtained with the other catalysts reportedin Table 1. The fact must also be added that the operation temperaturewindow with the catalyst Pt/s-50% MgO—CeO₂ (ΔT=190° C.) is, as far as weknow, the highest value ever reported for the reaction NO/H₂/O₂ with 5%H₂O in the supply stream.

Example 5

This example compares the activity (in terms of NO conversion, X_(NO))of the catalysts 0.1% wt Pt/s-50% MgO—CeO₂ (●), 0.1% wtPt/La_(0.5)Ce_(0.5)MnO₃ (▴) and 0.1% wt Pt/SiO₂ (▪) for the reactionNO/H₂/O₂ under NOx oxidation conditions with 5% vol H₂O in the supplyand in the range 100-400° C. The results indicate that the catalyst 0.1%wt Pt/La_(0.5)Ce_(0.5)MnO₃ has a higher activity than any other reportedto date for the reaction NO/H₂/O₂ [54]. 150 mg of each catalyst wereused and the supply consisted of 0.25% vol NO, 1% vol H₂, 5% vol O₂, 5%vol H₂O and 88.75% vol He. A flow velocity of 100 ml(STP)/min was usedgiving a GHSV of approximately 80,000 h⁻¹. As shown in FIG. 4, all thecatalysts present maximum conversion values of NO (X_(NO,max)) in therange 120-150° C. Nevertheless, the catalyst 0.1% wt Pt/s-50% MgO—CeO₂shows significantly higher conversion values than the catalysts 0.1% wtPt/La_(0.5)Ce_(0.5)MnO₃ and 0.1% wt Pt/SiO₂ at all temperatures in therange 100-400° C. As is clearly shown in FIG. 4 and in Table 1, thecatalyst 0.1% wt Pt/s-50% MgO—CeO₂ presents values of ΔT two and threetimes higher than those observed with the catalysts 0.1% wtPt/La_(0.5)Ce_(0.5)MnO₃ and 0.1% wt Pt/SiO₂ respectively (see Table 1).It can be seen that the catalyst Pt/SiO₂ exhibits virtually zeroactivity at temperatures higher than 250° C. The value of ΔT obtainedwith the catalyst 0.1% wt Pt/s-50% MgO—CeO₂ is the highest of all thosereported in the literature (Table 1). TABLE 1 Catalytic activity ofvarious Pt supported catalysts for the reaction NO/H₂/O₂ in thetemperature range 100-400° C. Reaction conditions ΔT R_(N2) ^(b)Tmax^(c) X_(NO,max) S_(N2) (%) S_(N2) Catalyst NO (%) H₂ (%) O₂ (%) (°C.) (μmol/s.g_(m)) (° C.) (%) (at X_(NO,max)) (%)^(d) X_(NO) (%)^(a)Ref. 1% Pt—Mo—Co—/Al₂O₃ 0.3 0.8 8.0 30 12.1 150 55 50 48.3 8.5 [47] 1%Pt/Al₂O₃ 0.05 0.2 6 40 10.1 140 50 60 12.9 10.0 [48] 1% Pt/TiO₂ 0.1 0.35.0^(f) 50 2.2 100 50 21 19.3 9.0 [49] 0.1% Pt/Al₂O₃ 0.25 1.0 5.0^(g) 45285.8 125 66 60 54.8 12.3 [54] 0.1% Pt/SiO₂ 0.25 1.0 5.0^(g) 55 240.1120 80 65 60.7 19.2 [59] 0.1% Pt/ 0.25 1.0 5.0^(g) 65 396.9 150 87 8786.4 42.8 [54] La_(0.5)Ce_(0.5)MnO₃ 0.1% Pt/ 0.25 1.0 5.0^(g) 190 418.2150 99 84 86.5 66.5 s-50%/MgO—CeO₂ ^(a)ΔT: Temperature range whereX_(NO) > X_(NOmax)/2, ^(b)maximum velocity of formation of N₂ (per gramof Pt), ^(c)Temperature at which the maximum conversion of NO ismeasured.${{\overset{\_}{S}}_{N_{2}} = \frac{\int_{T_{1}}^{T_{2}}{S_{N_{2}}{\mathbb{d}T}}}{T_{2} - T_{1}}},$^(d)S_(NS): mean value of selectivity to N₂: T₁ and T₂ are the lowestand highest temperatures,$\overset{\_}{X_{NO}} = {\frac{\int_{100}^{400}{X_{NO}{\mathbb{d}T}}}{400 - 100} = \frac{\int_{100}^{400}{X_{NO}{\mathbb{d}T}}}{300}}$respectively, where the activity is measurable; ^(e)X_(NO): meanconversion value of NO in the range 100-400° C.: ^(f)10% H₂O is presentin the supply, ^(g)5% H₂O is present in the suppl

Example 6

This example compares the selectivity to N₂ (S_(N2)) of the reactionNO/H₂/O₂ under NOx oxidation conditions as a function of temperature andin the range 100-400° C. obtained with the catalysts 0.1% wt Pt/s-50%MgO—CeO₂ (●), 0.1% wt Pt/La_(0.5)Ce_(0.5)MnO₃ (▴) and 0.1% wt Pt/SiO₂(▪). The experimental reaction conditions used in this example are thesame as those used in example 5.

As shown in FIG. 5, the catalysts Pt/s-50% MgO—CeO₂ andPt/La_(0.5)Ce_(0.5)MnO₃ present high values of selectivity to N₂ in therange 100-400° C. In particular, in the range 100-200° C. the catalystPt/s-50% MgO—CeO₂ shows values of selectivity to N₂ of between 82 and85%, while the catalyst Pt/La_(0.5)Ce_(0.5)MnO₃ presents values ofS_(N2) in the range 82-90%. In the range 250-400° C., the selectivity toN₂ is approximately constant at levels of 96 and 93% with the catalystsPt/s-50% MgO—CeO₂ and Pt/La_(0.5)Ce_(0.5)MnO₃, respectively. As reportedin Table 1, the catalyst Pt/s-50% MgO—CeO₂ presents a mean selectivityvalue to N₂ of 86.5%, which is practically the same as that obtainedwith the catalyst Pt/La_(0.5)Ce_(0.5)MnO₃ (86.4%). Much lowerselectivity values (50-65%) are obtained in the case of the catalystPt/SiO₂ which presents a mean value of S_(N2) of 60.7% (Table 1). Themean value S_(N2) obtained with the catalyst 0.1% wt Pt/s-50% MgO—CeO₂is the highest of all those reported in the literature to date (Table1).

Example 7

In this example, the stability of the catalyst 0.1% wt Pt/s-50% MgO—CeO₂is studied for the reaction NO/H₂/O₂/H₂O under NOx oxidation conditionsat 150° C.

The reaction conditions used in this example are the same as those inexample 5. FIG. 6 presents the variation in integral production velocityof nitrogen per gram of catalyst as a function of time for the catalysts0.1% wt Pt/s-50% MgO—CeO₂, 0.1% wt Pt/La_(0.5)Ce_(0.5)MnO₃ and 0.1% wtPt/SiO₂ in the stream. As shown in FIG. 6, the integral productionvelocity of N₂ obtained with the catalysts Pt/La_(0.5)Ce_(0.5)MnO₃ andPt/SiO₂ rapidly decreases during the first 2 hours in current. After thefirst 2 hours in reaction, the N₂ production velocity on these twocatalysts continues to fall though at a slower rate. On the other hand,the catalyst Pt/s-50% MgO—CeO₂ presents a practically constant N₂production velocity, even after 24 h in reaction. This is a result ofgreat importance since it is known that many NOx catalysts that havebeen tested undergo a deactivation with current time when water ispresent in the supply current [3].

Example 8

In this example, the stability of the catalyst 0.1% wt Pt/50% MgO—CeO₂for the reaction NO/H₂/O₂ in NOx oxidation conditions in the presence ofSO₂ in the supply is studied. The sulphur dioxide is one of the knownpoisons of many NOx catalysts [17].

The reaction NO/H₂/O₂/SO₂ is studied at 200° C. using 150 mg of thecatalyst 0.1% wt Pt/50% MgO—CeO₂ and a composition of the supply of0.25% vol NO, 1% vol H₂, 5% vol O₂, 23 ppm SO₂ and 93.75% vol He. A flowvelocity of 100 ml(STP)/min was used, which is equivalent to a GHSV ofapproximately 80,000 h⁻¹. FIG. 7 presents the NO conversion profileswith the time in the stream at 200° C. on the catalyst 0.1% wt Pt/50%MgO—CeO₂ when the sulphated and non-sulphated 50% MgO—CeO₂ support isused (see example 1). As can be seen, the catalyst Pt/MgO—CeO₂ with thenon-sulphated support is rapidly deactivated with the time in the streamand becomes completely deactivated after 20 h in reaction. Nevertheless,the catalyst Pt/MgO—CeO₂ with the support previously sulphated has acompletely different behaviour (Example 1, FIG. 7). This catalystpresents just a slight drop in NO conversion during the first 4 hours incurrent, at the same time showing a practically constant NO conversionafter the first 4 hours for a total time in stream of 24 h. This is aresult of excellent stability and of industrial importance since nostable catalysts in the reaction NO/H₂/O₂ in the presence of lowconcentrations of SO₂ have been reported. It is stated here that theconcentration of SO₂ in many industrial currents lies in the range 5-20ppm. So, the catalyst 0.1% wt Pt/50% MgO—CeO₂ with the pre-sulphatedsupport can find practical applications even in cases of maximum SO₂concentrations present in combustion streams.

Example 9

This example shows the effect of the partial pressure of hydrogen on thetemperature profile of the integral production velocity of N₂ in thereaction NO/H₂/O₂ under NOx oxidation conditions on the catalyst 0.1% wtPt/50% MgO—CeO₂. The experimental reaction conditions used in thisexample are the same as in example 3. FIG. 8 presents the temperatureprofiles of the N₂ production velocity obtained with the catalyst 0.1%wt Pt/50% MgO—CeO₂ (non-sulphated support) for the reaction NO/H₂/O₂ andusing hydrogen concentrations of 1 and 3% vol. As shown in FIG. 8, theintegral production velocity of N₂ substantially improves for allreaction temperatures when the H₂ concentration increases from 1 to 3%vol. In particular, an increase of two and four times is obtained at 150and 200° C. respectively, when the hydrogen concentration in the supplyincreases from 1 to 3% vol. This is a result of great importance sincewith the increase in hydrogen concentration the quantity of catalyst canbe regulated towards a minimum cost and desirable N₂ production yields.

Example 10

This final example describes the effect of the contact time (in terms ofthe W/F ratio) on the NO conversion with the catalyst 0.1% wt Pt/50%MgO—CeO₂. The supply consisted of 0.25% vol NO, 1.0% vol H₂, 5.0% vol O₂and 93.75% vol He. The variation in W/F was achieved by adjusting thequantity of catalyst (75-150 mg) and flow velocity (50-200 ml/min).

FIG. 9 shows the effect of the contact time on the NO conversion withthe catalyst 0.1% wt Pt/MgO—CeO₂ at 150° C. The NO conversion rapidlyincreases with the increase in contact time from 0.02 to 0.09 g.s/ml. Itis stated that the contact times of the NH₃—SCR reactors of NO oncommercial catalysts have typical values in the range 0.04-0.6 g.s/ml[1,55-57]. The low contact time required for obtaining high conversionsof NO on the catalyst Pt/MgO—CeO₂ indicates that the activity of thiscatalyst is sufficiently high for industrial application.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 presents the integral production velocities of N₂ per gram ofcatalyst as a function of the MgO content (x, % wt) in catalysts 0.5% wtPt/xMgO—CeO₂, obtained during the reaction NO/H₂/O₂ at 150° C. and 300°C. Reaction conditions H₂=1.0%, NO=0.25%, O₂=5%, W=0.1 g, GHSV=80,000h⁻¹, P_(tot)=1.0 bar.

FIG. 2 shows the influence of the content of Pt metal (% wt) on theintegral production velocities of N₂ during the reaction NO/H₂/O₂ underNOx oxidation conditions on the catalysts x % wt Pt/50% MgO—CeO₂ in therange 100-400° C. Reaction conditions H₂=1.0%, NO=0.25%, O₂=5%, W=0.1 g,GHSV=80,000 h⁻¹, P_(tot)=1.0 bar.

FIG. 3 shows the effect of the preparation method on the temperatureprofile of the integral production velocities of N₂ for the reactionNO/H₂/O₂ under NOx oxidation conditions on the catalyst 0.5% wt Pt/50%MgO—CeO₂. Reaction conditions H₂=1.0%, NO=0.25%, O₂=5%, W=0.1 g,GHSV=80,000 h⁻¹, P_(tot)=1.0 bar.

FIG. 4 compares the NO conversion temperature profiles (X_(NO)) of thereaction NO/H₂/O₂ under NOx oxidation conditions on the catalyst 0.1% wtPt/s-50% MgO—CeO₂ (●), 0.1% wt Pt/La_(0.5)Ce_(0.5)MnO₃ (▴) and 0.1% wtPt/SiO₂ (▪). Reaction conditions H₂=1.0%, NO=0.25%, O₂=5%, H₂O=5%,W=0.15 g, GHSV=80,000 h⁻¹, P_(tot)=1.0 bar.

FIG. 5 compares the temperature profiles of nitrogen selectivity(S_(N2)) of the reaction NO/H₂/O₂ under NOx oxidation conditions on thecatalysts 0.1% wt Pt/s-50% MgO—CeO₂ (●), 0.1% wt Pt/La_(0.5)Ce_(0.5)MnO₃(▴) and 0.1% wt Pt/SiO₂ (▪). Reaction conditions H₂=1.0%, NO=0.25%,O₂=5%, H₂O=5%, W=0.15 g, GHSV=80,000 h⁻¹, P_(tOt=)1.0 bar.

FIG. 6 compares the stability (in terms of integral reaction velocitiesof N₂) with current time in the catalysts 0.1% wt Pt/s-50% MgO—CeO₂ (●),0.1% wt Pt/La_(0.5)Ce_(0.5)MnO₃ (▴) and 0.1% wt Pt/SiO₂ (●). Reactionconditions H₂=1.0%, NO=0.25%, O₂=5%, H₂O=5%, W=0.15 g, GHSV=80,000 h⁻¹,P_(tot=)1.0 bar.

FIG. 7 compares the influence of the presence of SO₂ in the reactioncurrent on NO conversion (X_(NO)) during the reaction NO/H₂/O₂ under NOxoxidation conditions on the catalyst 0.1% wt Pt/50% MgO—CeO₂pre-sulphated (●) and non-sulphated (▴). Reaction conditions H₂=1.0% O,NO=0.25%, O₂=5%, SO₂=23 ppm, T=200° C., W=0.15 g, GHSV=80,000 h⁻¹,P_(tot)=1.0 bar.

FIG. 8 presents the effect of the partial pressure of hydrogen on theintegral production velocity of N₂ compared to the temperature for thereaction NO/H₂/O₂ under NOx oxidation conditions at a total pressure of1 bar on the catalyst 0.1% wt Pt/50% MgO—CeO₂. Reaction conditionsH₂=1.0%, NO=0.25%, O₂=5%, W=0.15 g, GHSV=80,000 h⁻¹ (●), and H₂=3.0%,NO=0.25%, O₂=5%, W=0.075 g, GHSV=320,000 h⁻¹ (▴).

FIG. 9 shows the influence of contact time (in terms of W/F) on NOconversion in the reaction NO/H₂/O₂ under NOx oxidation conditions onthe catalyst 0.1% wt Pt/50% MgO—CeO₂. Reaction conditions H₂=1%,NO=0.25%, O₂=5%, T=150° C., P_(tot)=1.0 bar.

CITED BIBLIOGRAPHY

-   1. G. Busca, L. Lietti, G. Ramis and F. Berti, Appl. Catal. B    18 (1998) 1.-   2. C. J. Pereria and K. W. Phumlee, Catal. Today 13 (1992) 23.-   3. A. Fritz and V. Pitchon, Appl. Catal. B 13 (1997) 1.-   4. T. A. Hewson, Jr., and J. B. Stamberg, Energy ventures analysis,    The Center for Energy and Economic Development, Midwest Ozone Group,    November 1995.-   5. R. I. Pusateri, J. R. Katzer and W. H. Monaque, AICHE J.    20 (1974) 219.-   6. T. Tabata, M. Kokitsu and 0. Okada, Catal. Today 22 (1994) 147.-   7. V. I. Parvulescu, P. Grange and B. Delmon, Catal. Today 46 (1998)    233.-   8. A. Obuchi, A. Ohi, M. Nakamura, A. Ogata, K. Mizuno and H.    Ohuchi, Appi. Catal. B 2 (1993) 71.-   9. R. Burch, P. J. Millington and A. P. Walker, Appl. Catal. B    4 (1995) 65.-   10. D. K. Captain, K. L. Roberts and M. D. Amiridis, Catal. Today    42 (1998) 65.-   11. R. Burch, J. A. Sullivan and T. C. Watling, Catal. Today    42 (1998) 13.-   12. R. Burch and A. Ramli, Appl. Catal. B 15 (1998) 63.-   13. M. D. Amiridis, K. L. Roberts and C. J. Perreria, Appl. Catal. B    14 (1997) 203.-   14. G. R. Bamwenda, A. Ogata, A. Obuchi, J. Oi, K. Mizuno and J.    Skrzypek, Appl. Catal. B 2 (1993) 71.-   15. E. A. Efthimiades, S. C. Christoforou, A. A. Nikolopoulos    and I. A. Vasalos, Appl. Catal. B: Envir. 22 (1999) 91.-   16. E. Seker, J. Cavatio, E. Gulari, P. Lorpongpaiboon and S.    Osuwan, Appl. Catal. A 183 (1999) 121.-   17. H. Hirabayashi, H. Yahiro, N. Mizuno and M. Iwamoto, Chem.    Lett. (1992) 2235.-   18. G. Zhang, T. Yamaguchi, H. Kawakami and T. Suzuki, Appl. Catal.    B: Envir. 1 (1992) L1519.-   19. A. Obuchi, A. Ohi, M. Nakamura, A, Ogata, K. Mizuno and H.    Ohuchi, Appl. Catal. B: Envir. 2 (1993) 71.-   20. W. Grunert et al., Chem. Tech. 47 (1995) 205.-   21. F. J. Janssen, in G. Ertl, H. Knözinger and J. Weitkamp (Eds.),    Handbook of Heterogeneous Catalysis, VCH, Weinheim, (1997) p. 1633.-   22. B. Rausenberger, W. Swiech, A. K. Schmid, C. S. Rastomjee, W.    Emgel and A. M. Bradshaw, J. Chem. Soc., Faraday Trans. 94(7) (1998)    963.-   23. R. Dumpelmannm, N. W. Cant and D. L. Trimm in A. Frennet and    J.-M. Bastin (Eds.)₃rd ICC and Automotive Pollution Control,    Brussels, 2 (1994) 13.-   24. K. Tomishige, K. Asakura and U. Iwasawa, J. Catal. 157 (1995)    472.-   25. W. C. Hecker and A. T. Bell, J. Catal. 92 (1985) 247.-   26. A. Hornung, M. Muhler and G. Ertl, Catal. Lett. 53 (1998) 77.-   27. T. P. Kobylinski and B. W. Taylor, J. Catal. 33 (1974) 376.-   28. S. J. Huang, A. B. Walters and M. A. Vannice, J. Catal.    173 (1998) 229.-   29. R. Burch and S. Squire, Catal. Lett. 27 (1994) 177.-   30. T. M. Salama, R. Ohnishi, T. Shido and M. Ichikawa, J. Catal.    162 (1996) 169.-   31. T. Tanaka, K. Yokota, H. Doi and M. Sugiura, Chem. Lett. (1997)    273.-   32. A. Lindsteld, D. Stromberg and M. A. Milh, Appl. Catal,    116 (1994) 109.-   33. D. Ferri, L. Formi, M. A. P. Dekkers and B. E. Nieuwenhuys,    Appl. Catal. B: Envir. 16 (1998) 339.-   34. J. R. Rostrup-Nielsen, Catal. Today 18 (1993) 305.-   35. I. Alstrup, J. Catal. 109 (1998) 241.-   36. S. T. Ceyer, Q. Y. Yang, M. B. Lee, J. D. Beckerle and A. D.    Johnson, Stud. Surf. Sci. Catal. 36 (1987) 51.-   37. I. Alstrup and M. T. Travers, J. Catal. 135 (1992) 147.-   38. T. B. Beebe, Jr., D. W. Goddman, B. D. Kay and T. J. Yates,    Jr., J. Chem. Phys. 87 (1987) 2305.-   39. I. Alstrup, I. Chorkendorff and S. Ullmann, Surf. Sci.    234 (1990) 79.-   40. H. J. Topfer, Gas Wasserfach 117 (1976) 412.-   41. S. Tenner, Hydrocarbon Processing 66(7) (1987) 42.-   42. A. T. Ashcroft, A. K. Cheetham, M. L. H. Green and P. D. F.    Vernon, Nature 352 (1991) 225.-   43. J. T. Richardson and S. A. Paripatyadar, Appl. Catal 61 (1990)    293.-   44. I. M. Bodrov and L. O. Apel'baum, Kinet. Katal. 8 (1967) 379.-   45. I. M. Bodrov and L. O. Apel'baum, Kinet. Katal. 5 (1964) 696.-   46. M. A. P{tilde over (e)}na, J. P. Gomez and J. L. G. Fierro,    Appl. Catal. A: Chemical 144 (1996) 7.-   47. B. Frank, G. Emig and A. Renken, Appl. Catal. B: Envir.    19 (1998) 45.-   48. R. Burch, M. D. Coleman, Appl. Catal. B. Envir. 23 (1999) 115.-   49. A. Ueda, T. Nakao, M. Azuma and T. Kobayashi, Catal. Today    45 (1998) 135.-   50. K. Yokota, M. Fukui and T. Tanaka, Appl. Surf. Sci    121/122 (1997) 273.-   51. M. Machida, S. Ikeda, D. Kurogi and T. Kijima, Appl. Catal. B:    Envir. 35 (2001) 107.-   52. R. Burch, P. J. Millington and A. P. Walker Appl. Catal B.    Envir. 4 (1994) 160.-   53. M. Fukui and K. Yokata, Shokubai, Catalysts and Catalysis    36 (1994) 160.-   54. C. N. Costa, V. N. Stathopoulos, V. C. Belessi and A. M.    Efstathiou, J. Catal. 197 (2001) 350.-   55. G. Centi, J. Mol. Catal. A: Chemical 173 (2001) 287.-   56. B. Ramachandran, G. R. Herman, S. Choi, H. G. Stenger, C. E.    Lyman and J. W. Sale, Catal. Today 55 (2000) 281.-   57. R. Khodayari and C. U. I. Odenbrand, Appl. Catal. B: Envir.    33 (2001) 277.-   58. K. Balakrishnan and R. D. Gonzalez, J. Catal. 144 (1993) 395.-   59. C. N. Costa, P. G. Sawa, C. Andronikou, G. Lambrou, K.    Polychronopoulou, V. N. Stathopoulos, V. C. Belessi, P. J. Pomonis    and A. M. Efstathiou, J. Catal. in press.-   60. S. Galvano and G. Paravano, J. Catal. 55 (1978) 178.-   61. S. Kikuyama, I. Matsukama, R. Kikuchi, K. Sasaki and K. Eguchi,    Appl. Catal. A: General 5480 1 (2001).-   62. S. Hodjati, C. Petit, V. Pitchon and A. Kiennemann, Appl. Catal.    B: Envir. 30 (2001) 247.-   63. V. C. Belessi, C. N. Costa, T. V. Bakas, T. Anastasiadou, A. M.    Efstathiou and P. J. Pomonis, Catal. Today 59 (2000) 347.

1. Catalyst based on platinum, with excellent activity, selectivity andstability for reducing NO to N₂ by means of using H₂ as reducing agentin the low temperature range 100-200° C. and in the presence of anexcess of oxygen (e.g. 5% vol), 5% vol H₂O and/or 20 ppm SO₂ in thesupply, which consists of Pt highly dispersed between 0.1 and 2% wt on amixed oxide support of Mg and Ce or any other precursor thereof, bymeans of damp impregnation or sol-gel techniques, wherein said catalystdemonstrates excellent activity (X_(NO)>40%) and selectivity levels toN₂ higher than 80% in a broad temperature range of 100-400° C., as wellas excellent stability of the reaction in the presence of 5% vol H₂O or20 ppm SO₂ in the supply using a surface contact time of 0.045 s. 2.Catalyst in accordance with claim 1, characterized in that the preferredmetallic content is of the order of 0.1% Pt highly dispersed on themixed oxide support of Mg and Ce or any other precursor thereof, bymeans of damp impregnation or sol-gel techniques.
 3. Catalyst inaccordance with claim 1, which consists of Pt supported on a mixed oxideof 50% MgO and 50% CeO₂.
 4. Catalyst in accordance with claim 1 whereinhexachloroplatinic acid is used as metallic precursor of the Pt. 5.Catalyst in accordance with claim 1 wherein other precursor compounds ofplatinum are used, e.g., platinum nitrate, platinum acetyl-acetonate,platinum chloride, etc.
 6. Catalyst in accordance with claim 1, whereinthe mixed oxide 50% MgO-50% CeO₂, previously pre-sulphated, is used as asupport.
 7. Catalyst for the reduction of nitric oxide with hydrogen inthe presence of excess oxygen, in accordance with claim 1, furthercontaining more than one of the following compounds: Pt, MgO, CeO₂,MgSO₄ and Ce₂(SO₄)₃, and which are all possibly formed in the catalystPt/s-MgO—CeO₂ under the conditions of preparation, calcination andreaction.
 8. Catalyst in accordance with claim 1, wherein a surfacecompound of magnesium is formed by interaction between the speciespresent in the gaseous phase under the reaction conditions and an oxideof magnesium present.
 9. Catalyst in accordance with claim 1, wherein asurface compound of cerium was formed by interaction between the speciespresent in the gaseous phase under the reaction conditions and an oxideof cerium present.
 10. Catalyst in accordance with claim 1, wherein asurface compound of platinum was formed by interaction between thespecies present in the gaseous phase under the reaction conditions andmetallic platinum present.
 11. Process for the preparation of thecatalysts claimed in claim 1 which comprises the technique of dampimpregnation which results in a catalyst with crystals of platinumdeposited on the MgO and CeO₂ phases in a proportion 1:1, impregnatedwith an aqueous solution containing the desired quantity of nitrateprecursor, evaporation of the excess water, drying, grinding and heatingat 300° C. in an air flow for 2 h; impregnating the resulting productwith an aqueous solution containing the desired quantity of sulphateprecursor, evaporating of water, drying, grinding and calcination at600° C. in an air flow for 2 h; impregnating the resulting mixedsulphated oxide with an aqueous solution containing the desired quantityof platinum precursor, evaporating of water, drying, grinding andheating at 500° C. under in an air flow until complete decomposition ofthe platinum precursor and reduction of the catalyst at 300° C. in H₂flow for at least for 2 h, thereby achieving a dispersion of 80% in thecatalyst 0.1% wt Pt/s-50% MgO—CeO₂.
 12. Process for the preparation ofthe catalysts claimed in claim 1 in reactions which consist of thereduction of nitrogen dioxide and/or mixture of nitric oxide andnitrogen dioxide to N₂ gas using hydrogen as reducing agent.
 13. Processfor the preparation of the catalysts in accordance with claim 1 in anykind of reactor employed industrially in such processes, e.g., fixed bedreactor, monolith type reactor, etc., for the reduction of nitric oxide,nitrogen dioxide or mixture of nitric oxide and nitrogen dioxide to N₂gas using hydrogen as reducing agent and in the presence and/or absenceof oxygen and/or water.